Multijunction solar cells are a proven route to higher efficiencies than possible with single junction devices. In a multijunction solar cell, cells of different bandgap are stacked upon one another with the highest bandgap at the top and the lowest bandgap at the bottom. This is illustrated in FIG. 1, with Eg1<Eg2<Eg3. Light with E>Eg3 is absorbed in the top cell, while light with E<Eg3 passes through the top cell and impinges on the middle cell, where light with Eg2<E<Eg3 is absorbed and light with E<Eg2 passes through the middle cell and finally light with Eg1<E<Eg2 is absorbed in the bottom cell. Light with E<Eg1 is not absorbed by any of the cells.
This strategy is a method to split up the solar spectrum into parts and capture light more optimally from each part of the spectrum with 3 different cells. The same strategy can be employed with just 2 cells or with more than 3 cells. The concept of the multijunction solar cell is first described in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960, entitled “Solar Energy Converter”.
The stack of cells is commonly made in one of two ways. In the first method, individual cells are made separately and mechanically stacked above one another. This provides maximum flexibility in the design of the individual cells. The second and very elegant method is to grow the cells monolithically above one another (with intervening tunnel junctions for electrical connection). To achieve high performance, the cells are ideally single crystal layers (grown epitaxially) and closely lattice matched to each other.
The current state of the art cells of this type are lattice matched Ge:(InGa)As:(InGa)P, which are now commercially available and used in space and concentrator PV applications and recently have demonstrated efficiencies under concentrated light exceeding 40%. In these devices the germanium (Ge) serves as a substrate and also as the bottom cell.
In monolithic multijunction solar cells, the cells are connected in series electrically which imposes the condition that the current flowing through them in operation must be the same. Ideally and preferably the cells are “current matched” by virtue of the solar illumination and the choice of the bandgaps, if there is latitude to adjust them.
Mechanically stacked cells are often used in 3-terminal operation (for 2 cells), since current matching is not readily obtained and separate electrical operation maximizes power output without imposing current matching conditions. However this approach increases costs at the system level, since 2 inverters are required for power conversion rather than one. U.S. Pat. No. 4,575,576 issued Mar. 11, 1986 entitled “Three Junction Solar Cell” shows that appropriate series and parallel connections of the bottom and top cells of monolithically grown devices allow voltage matching configurations for sets of cells that can be used in 2-terminal operation. Similarly, U.S. Pat. No. 6,353,175 issued Mar. 5, 2002 entitled “Two-terminal Cell-interconnected-circuits using Mechanically-stacked Photovoltaic Cells for Line-focus Concentrator Arrays” shows that appropriate series and parallel connections of the bottom and top cells of mechanically stacked cells allow voltage matching configurations for sets of cells that can be used in 2-terminal operation. Voltage matched systems have some disadvantages, including the complexity of system assembly and interconnection, as well as difficulty in achieving a sufficiently accurate voltage matching configuration, because the voltages are not easily adjustable.